Responses of soil ammonia-oxidizing microorganisms to simulated nitrogen deposition in a natural Castanopsis carlesii forest

Abstract

Subtropical region of China is one of the global hotspots receiving nitrogen deposition. Nitrogen deposition could affect the abundance and community structure of ammonia oxidizers including ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) and complete ammonia oxidizer (comammox Nitrospira), with consequences on soil nutrient cycling that are driven by microorganisms. There is limited understanding for the newly discovered comammox Nitrospira in the subtropical forest soils. Here, we investigated the effect of simulated N deposition on abundances of soil ammonia oxidizers in the Castanopsis fargesii Nature Reserve in Xinkou Town, Sanming City, Fujian Province, China. Soil samples were collected from the field plots which received long-term nitrogen deposition with different dosages, including: CK, no additional treatment; LN, low nitrogen deposition treatment, dosage of 40 kg N·hm-2·a-1; and HN, high nitrogen deposition treatment, dosage of 80 kg N·hm-2·a-1. After 8-year treatment, simulated N deposition decreased soil pH and organic matter content, and increased nitrate content. We failed to amplify the amoA gene of AOB in the tested soils. High nitrogen deposition increased the abundance of AOA, but did not affect the abundance of comammox Nitrospira clade A and clade B. The ratio of comammox Nitrospira to AOA decreased with N addition, indicating that N addition weakened the role of comammox Nitrospira in nitrification in the subtropical forest soils. However, there were strong non-specific amplifications for both comammox Nitrospira clades A and B, highlighting the demand for the development of high coverage and specificity primers for comammox Nitrospira investigations in the future. The abundance of comammox Nitrospira clade A was positively correlated with total nitrogen (TN) and NH4+ concentration, while that of clade B was positively associated with soil organic carbon (SOC), TN and NH4+ Concentration. Overall, our findings demonstrated that simulated N deposition increased the relative importance of AOA in nitrification in the natural Castanopsis carlesii forest soil. These findings could provide theoretical support in coping with global change and N deposition in these regions.